This invention relates to the art of powder coating resins and processes to use them.
It is well-known to use epoxy resins in powder coatings for articles. See, e.g., Tess, "Epoxy Resin Coatings", Epoxy Resins (2d Ed ) at 772-78 (Marcel-Dekker 1988); and 6 Encyclopedia of Poly. Sci. & Eng., "Epoxy Resins" at 375 (J. Wiley & Sons 1986), which are incorporated herein by reference. In summary, a solid epoxy resin is mixed with a solid curing agent, and optionally pigments, fillers, stabilizers, and the like. The composition is solid at room temperatures but has a glass transition temperature. It is applied as a powder to a cold or heated substrate, which is usually metal, by methods such as electrostatic spraying, fluidized bed or electrostatic fluidized bed. The powder is heated either by heat from the substrate or by an external heat source, to cause it to melt, flow together, fuse and cure to form a coating.
For most nonweatherable applications, the epoxy resin is a solid advanced resin of bisphenol A and either epichlorohydrin or a diglyicidyl ether of bisphenol A. The curing agent is typically dicyanodiamide, an aromatic amine, a phenoxy resin, a polyanhydride, a novolac or a carboxyl-terminated polyester. Polyester resins are particularly desirable in some applications because of their excellent appearance, resistance to overbake, and mechanical properties. Common polyesters used are relatively low molecular weight (as compared with polyesters used in fabrics) and branched to promote cross-linking. See, e.g., Verborgt, U.S. Pat. No. 4,065,438 (Dec. 27, 1977) and Marsiat, U.S. Pat. No. 4,085,159 (April 18, 1978), which are incorporated herein by reference.
The viscosity of the epoxy-polyester powder coating resins presents an area for improvement. The polyester resins have a relatively high viscosity (such as about 150 to 200 poise for a mixture of 70 weight percent polyester and 30 weight percent epoxy resin) under curing conditions. (Viscosity is measured using an ICI cone and plate viscometer at 175.degree. C.) Resins with a lower melt viscosity are desirable because they flow together and fuse better to provide a smoother and a more even appearance, and they are easier to process into a powder. It would be desirable to find a powder coating composition with substantially the same or better flow and/or appearance and lower viscosity under ordinary use conditions.